-LRB- CNN -RRB- -- What we know -- and psychiatrists have diagnosed for decades -- as schizophrenia may really be eight separate diseases , research published in The American Journal of Psychiatry suggests .

Scientists at Washington University in St. Louis analyzed the DNA of more than 4,000 people with schizophrenia . They matched any gene variations they found in the DNA with study participants ' individual symptoms . In doing so , they found several `` gene clusters '' that appear to cause eight distinct classes of schizophrenia , according to a statement from the university .

`` Complex diseases , such as schizophrenia , may be influenced by hundreds or thousands of genetic variants that interact with one another in complex ways , '' the study authors wrote in their introduction .

`` Genes do n't operate by themselves , '' Dr. C. Robert Cloninger , one of the study 's senior authors , explained in the statement . `` They function in concert much like an orchestra , and to understand how they 're working , you have to know not just who the members of the orchestra are but how they interact . ''

Schizophrenia is a chronic brain disorder that affects about 1 % of the population , according to the American Psychiatric Association . Symptoms can vary from hallucinations to disordered speech to attention and decision-making problems .

Past studies done on twins and families have shown that about 80 % of the risk for schizophrenia is inherited , the study authors say . A study published in July showed as many as 108 genes may be tied to the mental health disorder . But scientists have had trouble identifying specific genetic variations that put people at risk .

The Washington University researchers looked at instances where a single unit of DNA was altered , which is known as a single nucleotide polymorphism , or SNP . Then they identified 42 interactive SNP sets that significantly increased people 's risk of schizophrenia , according to the study .

In other words , if study participant Bob had Gene Cluster X , he was 70 % more likely to have schizophrenia than study participant Fred who did n't have that cluster of genes . In some cases , certain gene clusters were matched with close to a 100 % increase in risk .

`` In the past , scientists had been looking for associations between individual genes and schizophrenia , '' co-author Dr. Dragan Svrakic said in the statement . `` What was missing was the idea that these genes do n't act independently . They work in concert to disrupt the brain 's structure and function , and that results in the illness . ''

The idea that schizophrenia is not one single disorder is not really new , says Dr. Charles Raison , a professor of psychiatry at the University of Arizona . It 's similar to the way doctors use the term `` breast cancer '' to describe several different diseases that cause tumors in the breasts .

`` Schizophrenia is probably 80 different diseases , '' Raison says . `` All psychiatric conditions likely share this heterogeneity . ''

There are only so many ways that certain malfunctions in your genetic code can manifest , Raison says . There may be 10 separate gene mutations , but they might only express themselves as one or two symptoms . So what 's causing hallucinations in one person might be different than what 's causing them in another .

So why are scientists trying to separate out the different schizophrenia disorders ? Two reasons , Raison says : to help predict who might get schizophrenia , and to help treat it more efficiently .

Take , for example , pleurisy , which is a condition where the liquid around your lungs becomes inflamed . Several things can cause pleurisy , including a viral infection , pneumonia or cancer . If you have a drug that treats pneumonia , it 's going to help only a certain percentage of patients with pleurisy . But if you know that your patient 's pleurisy is caused by cancer , you 'll find a different course of treatment .

The same could hold true for schizophrenia and other mental health conditions , Raison says .

`` In psychiatry land we 're still stuck with pleurisy , '' he says . `` They 're descriptions of symptoms , and we only have a vague idea of the underlying causes . ''

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Researchers say schizophrenia is not one disorder , but eight

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Genetic variations cause eight classes of schizophrenia

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Study was published in The American Journal of Psychiatry